DISTRIBUTION OF GEOLOGIC FIELD RELATIONS 307 



the entire region maintain a general similarity in direction of strike to 

 the northwest. In other words, they apparently lie in the northwest 

 quadrant, but naturally exhibit varying degrees of strike within the limits 

 of the quadrant. An exception to this was noted in the dike at Loch 

 Laird, which diabase has been intruded between nearly vertical beds of 

 calcareous shale belonging ip the Shenandoah group of limestones (see 

 plate 7). Whether or not this represents one of a series of outcrops of 

 the same dike across the valley in that latitude is not known. Farther' 

 north a single occurrence of this kind is always accompanied by a number 

 of others distributed in relation to it in the manner described above. A 

 second exception is noted in some of the dikes of the Monterey district in 

 Highland County, where possibly they are aligned in a north-northeast 

 direction. 



In width the dikes vary from 4 feet up to 80 feet, with a probable 

 average of about 20 feet. They may be readily traced by the character 

 of their residual soil and by the presence of abundant large and small 

 weathered boulders, so thickly strewn over the surface along the direction 

 of strike as to make agriculture impossible in many cases (see plates 8 

 and 9). Usually the boulders of the basic rocks (diabase, camptonite, 

 and teschenite) exhibit well rounded form from spheroidal weathering 

 (see plate 8, figures 1 and 2). The basic dikes as a rule exhibit but 

 slight or no topographic expression, while the more acid rock nepheline 

 syenite occupies the crests of ridges and is exposed both by a line of low 

 weathered reefs and loose masses that are only partially rounded (see 

 plate 9, figure 1). 



With the single exception noted above of diabase, the distribution of 

 the dikes of igneous rocks and their structural features bear no evident 

 relation to the dominant Appalachian structure. The lines of outcrops, 

 as well as the individual outcrops themselves, strike in a direction nearly 

 normal to the axes of the structure of the Paleozoic sediments. As may 

 be seen by consulting the accompanying map, figure 1, the dikes maintain 

 fairly close parallelism with each other, and, moreover, they sustain the 

 same relations to the inclosing sedimentary rocks regardless of their 

 lithologic character and the attitude of the beds. 



To the writers it seems hardly reasonable to assume Mini the above ar- 

 rangement of the dikes — their direction of strike and the structure of the 

 inclosing rocks — is fortuitous. It is rather to be inferred thai their gen- 

 eral uniformity in direction of strike is (he expression of some structural 

 feature developed in the Shenandoah group of limestones and other later 

 formations of. the region ;ii some period postdating the folding and fault- 

 ing of the rocks from the Appalachian revolution which closed the Paleo- 



